The present invention relates to gloss sensors for measuring the surface characteristics of paper sheets and more particularly to measurement of gloss using a single device which provides compensation for dirt buildup.
One of the parameters used in determining the quality of a surface is the surface luster or the gloss of the surface. For example, in paper production the various grades of paper having different surface gloss are produced to suit various applications. During paper production, it is desirable to periodically or continuously measure the gloss of the surface of the paper to ensure that the paper surface has the desired gloss.
Typically, the surface gloss of the paper is measured using a gloss gauge during the last step of paper production before the finished paper is packaged as rolls and shipped. The rolls of paper are then shipped to paper products manufacturers, who process the paper sheet in accordance with the intended use.
Devices for determining the gloss of paper surfaces utilize an optical system which measures the intensity of a beam of light reflected from a paper surface. Typically, the gloss of the paper surface is determined by comparing its reflectance to the reflectance of a known gloss standard, such as a glass tile having a polished surface with a known gloss.
Specifically, in measuring the reflectance of the paper surface, light is projected onto the surface, and a sensor which is responsive to the intensity of light is positioned to measure the intensity of the light reflected from the paper surface. The gloss gauge measures the reflectance of the tile surface in the same manner by substituting the tile surface for the paper surface. The reflectance of the paper surface is referenced to the reflectance of the tile surface, thereby providing a measurement of the gloss of the paper surface. In practice, the reflectance measurement of the tile surface is periodically performed, off-sheet and between scans, as the gloss gauge scans back and forth across the paper surface. The gloss gauge is calibrated during each such measurement with the known reflectance of the tile surface.
Two gloss sensor standards have been developed in the industry under this technique. The first standard, outlined under DIN 54502, for regular gloss measurements specifies that the measurements are to be taken using an angle of 75xc2x0 for the incident light beam from a line perpendicular to the measured surface. For high-gloss measurements, measurements are taken using an angle of 45xc2x0 for the incident light beam from a line perpendicular to the surface to be measured. If both measurements are to be made, two separate and distinct sensors are generally used. The second standard, outlined under TAPPI T480, specifies that the measurements are to be taken only using an angle of 75xc2x0 for an incident light beam from a line perpendicular to the measured surface.
One problem associated with the available gloss sensors is their inability to function properly if the sensor windows become dirty. For example, during production a fine paper dust is produced. This fine paper dust coats the reference tile or the sensor windows; thus, when the sensor is reset by scanning the reference tile, or if a light beam is passed through dirty sensor windows, an inaccurate reading is produced. Therefore, the gloss of the paper will not adequately be detected.
Therefore, there is a need for a gloss sensor that is capable of producing accurate gloss measurement results independent of dirt buildup during production.
There is also a need for a gloss sensor that is more compact and that utilizes less parts, requiring less maintenance than presently-available sensors.
In accordance with one aspect of the present invention there is provided a gloss sensor for optically measuring the gloss of a surface. The gloss sensor includes a light source, a first collimator for receiving the light from the light source and arranged to form a collimated light beam, wherein the collimated light beam is emitted from a first end of the collimator. The gloss sensor further includes: a first detector within the collimator for developing a reference signal; a beam splitter disposed adjacent the first end of the collimator for dividing the collimated beam into a first beam and a second beam, the second beam being received by a reflection device; a first mirror, positioned adjacent a first window, the first mirror adapted to reflect the first beam onto a surface to be measured and further adapted to receive the second beam reflected from the reflection device. A second mirror is positioned adjacent a second window. The second mirror receives the first beam reflected from the surface to be measured and the second beam reflected by the first mirror through the first window and the second windows. The second mirror is positioned at and angle to reflect the second beam back to the reflection device. A second collimator is disposed adjacent the first collimator, wherein the second collimator is positioned to receive the first beam reflected from the second mirror. A second detector is located within the second collimator and is adapted to receive the first beam. A third detector disposed adjacent the reflection device is adapted to receive the second beam.
In accordance with one aspect of the present invention the gloss sensor for measuring the gloss of a surface includes an illumination source and at least two collimators, wherein the illumination source is disposed within one of the two collimators. A first detector is disposed within the first collimator, the first detector adapted to receive a beam emitted from the illumination source. The gloss sensor further including a beam splitter that divides a beam emitted from the illumination source into two separate beams and a means for reflecting the two separate beams. A second detector disposed within the second collimator receives the one of the two beams from the beam splitter with a third detector positioned to receive the other second beam.
In accordance with another aspect of the present invention there is provided a gloss sensor for optically measuring the gloss of a sample surface. The gloss sensor includes: a light source; a first collimator for receiving the light from the light source to form a collimated light beam, wherein the collimated light beam is emitted from a first end the first collimator; a first detector located within the collimator for developing a reference signal and a beam splitter disposed adjacent the first end of the collimator, wherein the beam splitter divides the collimated beam into a first beam and a second beam, the second beam being received by a reflection device. A first mirror positioned adjacent to a first window, reflects the first beam onto a surface to be measured, the first mirror further adapted to receive the second beam reflected from the reflection device. A second mirror positioned adjacent a second window receives the first beam reflected from the surface to be measured and the second beam reflected by the first mirror through the first window and the second window, wherein the second mirror is positioned at and angle to reflect the second beam back to the reflection device. A second collimator disposed adjacent the first collimator is positioned to receive the first beam reflected from the second mirror. A second detector disposed within the second receives the first beam and produces a measurement signal. A third detector disposed adjacent to the reflection device, receives the second beam and produces a dirt compensation beam, and a signal correction means for producing a gloss reading by correcting the measurement signal with the reference signal and the dirt buildup signal.